1. Field of the Invention
This invention relates in general to a sealing glass for the production of glass-metal seals and, more particularly, to glass seals which can be used for battery headers in batteries having a lithium electrode. The header consists of inner and outer conductors for internal attachment of the anode and cathode and external attachment of the battery leads. The conductors are electrically isolated from each other by a glass insulator (a glass seal), which also functions as part of the hermetic battery seal.
2. Description of the Related Art
In storage batteries with a lithium electrode, the cell housing is hermetically sealed. A seal is also provided between the housing and the terminals by a glass insulator in the cell head. The glass insulator is fused to the metal housing (outer electrode) and the outwardly extending inner metallic conductor, which, in general, is connected to the cathode or is an extension of the cathode itself.
The service life and shelf life of these cells depend in large part on the stability of the glass insulator, i.e., the stability of the sealing glass, since the sealing glass is exposed to especially strong corrosive attack by the contents of the cell. Further, the thermal expansion of the glass and the metal of the housing must closely match for long service life of the glass seal. Corrosion of the glass seal can be caused by lithium; by the depolarizer (the cathode material), such as SO.sub.2, SOCl.sub.2, POCl.sub.3, SeOCl.sub.2, SO.sub.3, VOCl.sub.3, CrO.sub.x, Cl.sub.2, SO.sub.2 Cl.sub.2, NO.sub.2 Cl, NOCl, NO.sub.2, S.sub.2 Cl.sub.2, S.sub.2 Br.sub.2, and their mixtures; by solid depolarizers, such as HgO, HgCrO.sub.4, manganese dioxide; and, in general, by metal halides, oxides, chromates, bichromates, permanganates, periodates, molybdates, vanadates, chalcogenides, and their mixtures. The depolarizers most frequently used are SOCl.sub.2, SO.sub.2, and CrO.sub.x. Corrosion of the glass seal can also be caused by the electrolytes in which the conducting salts are dissolved. Such electrolytes are, for example, tetrahydrofuran, propylene carbonate, dimethyl sulfate, dimethyl sulfoxide, gamma-butyrolactone, dimethyl carbonate, methyl formate, butyl formate, dimethoxyethane (DME), acetonitrile, n-nitrosodimethylamine and dimethylformamide. Light metal salts, especially lithium salts such as halides, perchlorates, tetrachloroaluminates, fluoroborates, hexafluorophosphates, hexafluoroarsenates, and clovoboranates, are used as conducting salts. The electrolyte conducting salt combinations most frequently used commercially at the present time are lithium tetrachloroaluminate in thionyl chloride or dimethoxyethane or propylene carbonate.
The glass seal also serves as electric insulation. Thus, the function is the glass seal is three-fold, providing (a) corrosion resistance, (b) electrical insulation, and (c) hermetic sealing of the cell.
The metals suitable for the cell housing or for the shunts are determined by the cell components with which they come into contact. Such metals compatible with lithium are, e.g., copper; iron, steel, and all iron alloys; nickel and nickel alloys such as KOVAR, INCONEL or MONEL, and especially iron-nickel alloys; titanium; tungsten; molybdenum; vanadium; niobium; tantalum; etc. Metals compatible with SO.sub.2, SOCl.sub.2, and SO.sub.2 Cl.sub.2 are, e.g., titanium, tantalum, vanadium, tungsten, niobium, molybdenum, and nickel alloys such as KOVAR, INCONEL, MONEL, etc.; and metals resistant to silver chromate are, e.g., titanium, molybdenum, vanadium, tantalum, tungsten, chromium, and stainless steel.
The lithium cells now most widely used have housings and covers of nickel-plated steel or high-grade steel, as well as shunts of high-grade steel or sealing alloys on a Ni-Fe base. The "shunt" is the current conductor, normally the center pin. The "sealing alloys" are alloys which are "sealable" with the glass seal; thus, they have an appropriate coefficient of thermal expansion.
Since the service life of these cells essentially depends on the stability of the sealing glass of the glass insulator, there have been many attempts to improve the stability of the sealing glass.
To improve the service life of an insulator consisting of a conventional silicate or borosilicate sealing glass, DE-PS 29 04 396 discloses coating the glass surface exposed to the cell interior with a protective layer. This procedure is expensive, and there is always the danger that the covering will leak. Therefore, it has also been attempted to improve the solder glass itself, and numerous glasses have been tested to determine their suitability as a sealing glass in lithium cells. In the Sandia Report SAND 83-2314, pages 7-20, silicate, aluminoborate, phosphate, calcium aluminate, and alkali borate glasses are described. The silicate glasses have an SiO.sub.2 content of over 40% by weight; they are susceptible to corrosion by the cell contents. In addition, the silicate glasses are impractical because of the mismatch of the coefficient of the thermal expansion relative to Ni/Fe alloys.
The aluminoborate glasses have only a slight stability against crystallization (devitrification stability) and insufficient corrosion resistance. They also lack a coefficient of thermal expansion which matches that of Ni/Fe alloys. The high cost of the aluminoborate glasses due to the relatively large amount of lanthanum therein makes this glass even more unsuited for use as a glass seal around terminals of storage batteries.
The phosphate glasses are largely inert relative to the cell contents; however, they are very sensitive to crystallization. Also, these glasses do not have a coefficient of thermal expansion which closely matches Ni/Fe alloys.
The calcium aluminate glasses, which essentially consist of CaO (up to 45% by weight), are actually very resistant to the battery contents; however, in practice, they are not useful because of their tendency to crystallize and also their lack of resistance to hydrolysis. These glasses were found to decompose in a stability test according to ISO 719.
The lithium borate glasses are actually largely inert relative to the cell contents; however, they are used only for experiments due to their tendency to crystallize making them unsuitable for use in lithium cells.
A sodium-resistant sealing glass is known from DE-OS 29 42 538, which is characterized by a high proportion of B.sub.2 O.sub.3, and a low proportion of SiO.sub.2. A Cs-vapor-resistant lithium-free glass with a high alkaline-earth content is described in CA 104,114783d/SU-PS 1189824; however, there is no mention of its suitability for use in lithium cells.
In J. Mater, Res., Vol 2, No. 2, p. 182 (1987), a number of different glasses are examined for their suitability as corrosion-resistant sealing glass for terminals in lithium batteries. After the examination of numerous glasses, the authors came to the conclusion that the corrosion of glass can be eliminated by the use of silicate-free glasses such as aluminoborate or borate glasses. However, such glasses, do not exist which are useful in practice.
Therefore, an object of the invention is to find a sealing glass for cells having corrosive contents, especially lithium cells. The sealing glass should have both a good resistance to the contents of the cell, and a coefficient of thermal expansion which is matched to a sufficient extent with that of the metals used for the cell housing and shunt, especially Ni/Fe alloys. Further, the sealing glass should have a sufficient hydrolytic resistance since in the course of their production the cells are washed and the glass-to-metal seal must survive the washing without loss of quality.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.